1. Field of the Invention
This invention relates to an improved process for separating a hydrocarbon-bearing feed gas which contains methane and lighter components, (not necessarily all hydrocarbon components), C.sub.2 (ethylene and ethane), and heavier hydrocarbon components into two fractions. The first fraction contains predominantly methane and lighter components and the second fraction contains the recovered desirable C.sub.2 and heavier components. More particularly, this invention relates to a process and apparatus wherein the yield of C.sub.2 's is increased or alternatively energy consumption is reduced for a given C.sub.2 recovery.
2. The Prior Art
Hydrocarbon-bearing gas may contain lighter components (e.g., hydrogen, nitrogen, etc.) methane, ethane, and/or ethylene, and a substantial quantity of hydrocarbons of higher molecular weight, for example, propane, butane, pentane and often their unsaturated analogs. Recent changes in ethylene/ethane demand have created increased markets for ethylene/ethane and have created a need for more efficient processes which yield higher recovery levels of this product. In more recent times, the use of cryogenic processes utilizing the principle of gas expansion through a mechanical device to produce power while simultaneously extracting heat from the system have been employed. The use of such equipment depends upon the pressure of the gas source, the composition of the gas and the desired end results. In the typical cryogenic expansion-type recovery processes used in the prior art, a gas stream under pressure is cooled by heat exchange with other streams of the process and/or external sources of cooling are employed such as refrigeration systems. As the gas is cooled, liquid is condensed and is collected and separated so as to thereby obtain desired hydrocarbons. The high-pressure liquid feed is typically transferred to a demethanizer column after the pressure is adjusted to the operating pressure of the demethanizer. In such fractionation column the liquid feed is fractionated to separate the residual methane and lighter components from the desired products of ethylene/ethane and heavier hydrocarbon components. In the ideal operation of such separation processes, the vapor leaving the process contain substantially all of the methane and lighter components found in the feed gas and substantially no ethylene/ethane or heavier hydrocarbon components remain. The bottom fraction leaving the demethanizer typically contains substantially all of the ethylene/ethane and heavier hydrocarbon components with very little methane or lighter components which is discharged in the fluid gas outlet from the demethanizer.
A patentability search was conducted on the present invention and the following references were uncovered.
Inventor U.S. Pat. No. Issue Date Harandi 4,664,784 5/12/1987 Buck et al 4,895,584 1123/1990 Campbell et al 5,771,712 9/01/1998 Wilkinson et al 5,699,507 6/30/1998
U.S. Pat. No. 4,664,784--Issued May 12, 1987
M. N. Harandi to Mobil Oil Corporation
In a reference directed to fractionation of hydrocarbon mixtures, teachings are found on column 4, line 32 et sequitur re: a zone (81) wherein a descending liquid heavy-ends portion contacts an ascending vaporous light-ends portion so as ". . . to aid in heat transfer between vapor and liquid." (column 4, line 44).
U.S. Pat. No. 4,895,584--Issued Jan. 23, 1990
L. L. Buck et al to Pro-Quip Corporation
A reference that claims an improved process for hydrocarbon separation and teaches supplying of the liquid recovered from the light-ends fractionation column to the heavy-ends fractionation column and directing part of the (C.sub.2 containing) liquid from a first step into intimate contact with a second residue, which liquid provides additional liquefied methane which acts with the partially condensed second residue as a direct contact refrigerant to thereby condense C.sub.2 and heavier comprising hydrocarbons while methane itself is evaporated in the light-ends fractionation column.
On column 1, lines 56-67 the following teachings are found: ". . . feed gas is first cooled and partially condensed and delivered to a separator to provide a first residue vapor and a liquid containing C.sub.2 . . . Part of the liquid containing C.sub.2 from the separator may be directed into a heavy-ends fractionation column wherein the liquid is separated into a second residue containing lighter hydrocarbons and C.sub.2 containing products. A part of the first residue vapors with at least part of the partially condensed second residue are counter currently contacted and commingled in a light-ends fractionation column (emphasis added) . . . "
On column 2, lines 1-10 the following teachings are found: "The liquids recovered from the light-ends fractionation column are then fed to the heavy-ends fractionation column as a liquid feed. A portion of the liquids containing C.sub.2 from the separator is fed into intimate contact with the second residue prior to discharging the commingled liquids and gases into the light-ends fractionation column to thereby achieve mass and heat transfer (emphasis added) to thereby liquefy a higher percent of the C.sub.2 and heavier hydrocarbon components while the methane is vaporized" (column 2, lines 1-10).
The following Elcor Corporation references describe the recovery of C.sub.3 and heavier hydrocarbons via processes wherein counter-current contact of a stream drawn from a deethanizer with a stream in a separator/absorber takes place:
U.S. Pat. No. 5,799,507--Issued Sep. 1, 1998
J. D. Wilkinson et al to Elcor Corporation
See column 4, line 2 re: ". . . liquid portion of expanded stream commingles with liquids falling downward from the absorbing section . . . " l.o.w., the stream (36) from the deethanizer (17) flows through heat exchanger (20) to become stream (36a) which flows into the upper section of separator (15) where it ". . . contacts the vapors rising upward through the absorption section" (column 5, lines 3-4).
U.S. Pat. No. 5,771,712--Issued Jun. 30, 1998
R. E. Campbell et al to Elcor Corporation
This reference teaches essentially the same as Wilkinson et al.
None of the foregoing patents discussed above embody the present invention.